1. Field of the Invention
This invention relates generally to an additive to a synthetic resin mixture of formulation for use in the production of plastic lenses, and more specifically, to a group of silanes found suitable for this purpose, with the product being useful in lenses used for eye glasses.
2. Description of the Prior Art
Lens devices fabricated from plastic resinous substances have been made in the past, with the lens devices traditionally having been made from polycarbonate resins such as diethylene glycol bis(allyl carbonate) which is known commercially in its monomeric form as allyl diglycol carbonate and in either its monomeric or polymerized form, as CR-39. It therefore should be understood that these terms are interchangeable in the art, and that the use of any of these terms commonly identifies the same monomer.
Because of the clarity, high strength and high impact resistance of CR-39 in its polymerized form, it is suitable for use in plastic lenses for eye glasses. The wide acceptance is also due to the property of high resistance to discolorization and also its resistance to physical warping or distortion. A more complete description of this type of monomer and the manufacture of this type of plastic lens can be found in the Beattle U.S. Pat. No. 2,542,386. Still another material used for lenses for eye glasses is described in the Emerson et al U.S. Pat. No. 3,297,422 which suggests the use of a starting material such as methyl methacrylate monomer. Other materials, such as copolymers of methyl methacrylate and CR-39 are known, one of which is shown in the Bond U.S. Pat. No. 3,872,042.
While the manufacture of formulation of a composition of a monomer for preparing a lens is well known, the process for industrial production of a lens comprised of a polymer consisting of two or more monomers has been extremely difficult due to the problems generated due to erratic or partial release of the plastic lens from the glass mold. The problem of release is normally resolved through control of the adherence of the surface of the plastic lens to the surface of the mold during curing within the mold and has been found to be dependent on many processing parameters or variables such as the cleanliness of the mold, the composition of the monomer mixture, the length of the curing cycle, the temperature of the curing cycle, the surface irregularities of the mold, and the shape and curvature of the mold as well as the techniques of those involved in the injection or casting process. To obtain high yields and thus make industrial production possible at acceptable rates, the adherence of the surface of the finished plastic lens product to the surface of the mold must be sufficient to firmly hold or retain the plastic lens against the mold surface through the curing stage, but yet sufficiently weak to allow cohesive separation following cure.
In the past, control of the optical quality of the lens devices was obtained by control of the monomer mixture. When it was determined that the monomer mixture would produce a lens of a quality sufficient for optical use, then an examination of the casting, molding, or other production steps to be utilized revealed that the pre-release of the molded product from the surface of the mold (i.e., the release of the lens from the surface of the mold prior to the complete curing of the lens) would normally yield a product of unacceptable optical quality.
One prior art method used to control the release is the use of mold surface lubricants or release agents such as stearic acid. Another mold lubricant is commonly known in the trade as Ortholeum 162, its Code Name, Ortholeum 162 being a mixture of mono and dialkylphosphates with a straight chain alkyl group of C16 to C18. However, the known mold lubricants of this type have not provided the consistent control of the release of the lens from the mold when used with copolymer or multipolymer formulations and, consequently, the yield (i.e., the number of lenses from a particular run that can be used in the eye glasses) has been poor. That is, there is a tendency of the lens to release erratically from the mold by either premature release from the mold or complete adherence. If the lens releases prematurely, i.e., before the curing is completed, the lens is usually unsuitable for optical purposes. On the other hand, if the lens does not release easily subsequent to curing, one can destroy both the plastic lens and the mold by attempting to open the mold by prying the plastic lens from the mold. In either case, the plastic lens product may be unsuitable for use in eye glasses. The strange part of this phenomena of release is that the use of additives which are known to act as lubricants by decreasing the adherence of the surface of the mold to the surface of the plastic do not yield consistent results. That is, the amount of lubricant can be held constant with identical polymers and in one case, the lens may not release from the mold and in another case, the lens may release from the mold prior to completion of the curing. It is believed that such irregular results are due to variation in technique processing parameters, mold variations or other undetermined variables.
A terpolymer has been used for plastic lenses that contains acrylic acid or methacrylic acid in minor amounts to produce a terpolymer that has the desired optical qualities for use in eye glasses yet provides high yields because the lens consistently release from the surface of the glass mold at the proper time or can be easily forced to release. This terpolymer is more fully described in co-pending application Ser. No. 441,645, filed Feb. 11, 1974, now patent 3,944,637, and assigned to the same assignee as the present invention. The consistent release was somewhat surprising in that the acrylic and methacrylic acid were believed to increase the adherence of the surface of the glass mold to the surface of the plastic lens. One aspect of the problem was the tendency of the surface of the mold to adhere too strongly to the surface of the plastic lens. Nevertheless, the addition of the third monomer in the polymeric mass has been found to increase the hardness of the plastic lens as well as to solve the problems of pre-release or over-adherence of the lens to the mold. However, while this process and terpolymer has proven successful for mass production of lenses under carefully controlled conditions, the transition of the process to full scale industrial process has not been easily accomplished because the expected yields, under industrial production conditions, have not matched those obtained under laboratory conditions.
One of the problems is that with lesser amounts of CR-39, the methacrylic acid begins to polymerize first, thus producing a block polymer rather than a copolymer. Consequently, the amount of methacrylic acid used must be limited to avoid the graying of the monomer which occurs due to block polymer formation. For example, with 98 parts CR-39, the maximum amount of methacrylic acid that can be used without graying is about 1.1%. With a limited amount of methacrylic acid and because of other factors, a normal release of lenses from the molds could not be obtained.
It was believed that the transition from mass production of plastic lenses under carefully controlled conditions to normal industrial production of plastic lenses in which plastic lenses are cast, cured and polished in successive processing steps would appear to be a straight forward step as long as the variables involved in the casting and curing process were observed and taken into consideration. Surprisingly, this has not been the case. It has been discovered that in spite of the close control of the many known variables, the industrial production of plastic lenses has been very costly because of that phenomenon known as "pre-release." As a general rule, it has been found that if the percentage of product subjected to pre-release exceeds 30% of the total cast lenses, then the process of producing lenses normally becomes uneconomical. On the other hand, to make the process economical, it is also necessary to be able to fabricate about 200 castings from a single mold. However, as each lens in subject to operations of casting, curing, and subsequently removal, these operations have a deteriorating effect on the mold and, consequently, reduces the number of lenses which can be cast from a single mold. Thus, under industrial production, where the molds must be continually reused to make the process economically sound for large quantity production of plastic lenses, some sort of balance must be achieved between the number of lenses that pre-release and the length of the mold life. If the pre-release percentage is too low, the mold life decreases too rapidly; while if the pre-release percentage is too high, the number of acceptable lenses produced becomes unacceptable. The control of the known variables has not been sufficient to achieve a proper balance between the percentage of lenses that pre-release and the number of castings per mold. However, with the controlled addition of certain silanes, it has been found that a dominant production variable is provided which will allow casting of plastic lenses of both copolymers and terpolymers with the desired release characteristics.